The present invention relates to a method of manufacturing an ink jet print head of a shear mode type using a piezoelectric element and having a high image quality, a high speed, and a high density. An ink jet print head of a shear mode type has ink channels formed in a polarized piezoelectric substrate by cutting and grinding, and has electrodes formed on the partition walls of the channel grooves; it jets ink drops when an electric field is applied to these electrodes to cause the partition walls of the grooves to be deformed to become dog-legged shape by a shearing force, to apply a pressure to ink in the grooves. For this ink jet print head, electrodes in the grooves and connection electrodes outside the grooves which transmit a signal to the electrodes are necessary. The present invention is a new method of forming the electrodes and the connection electrodes, using a laser and an electroless plating process without using a conventional photosensitive resist.
A fundamental structure of a conventional print head will be explained by referring to FIG. 14. For example, according to the publication of the examined patent application H6-61936, a polarized piezoelectric substrate is coated with a photosensitive resist by a spinning method. In another way, a photosensitive dry film is laminated on it. Subsequently, mask exposure and development is carried out to form a pattern of ink channels. Next, grooves are formed by cutting and grinding along this pattern. Each of these grooves is not uniformly formed over the whole length, but by stopping the cutting and grinding operation on the way, a straight groove portion 100, a shallow groove portion 101 having its depth made gradually smaller, and an uncut portion 102 which is not cut and ground at all are successively formed. Subsequently, aluminum is coated by evaporation coating from an oblique direction to the grooves, to cause electrodes to be formed on the partition walls of the grooves, connection electrodes to be formed on the shallow groove portions and uncut portions. By connecting the ends of these connection electrodes to a drive circuit by flexible cables, a signal can be sent to the electrodes. However, because these shallow groove portion and uncut portion is not scarcely deformed by the application of an electric voltage, they cannot contribute to the jetting of ink drops. While this ink jet print head is driven, owing to the hysteresis loss of the piezoelectric element caused by the application of a high-frequency signal voltage, it generates heat remarkably because the electric voltage is applied also to these portions that do not scarcely contribute to ink jetting. In particular, because the uncut portions 102 do not come into direct contact with ink and is not cooled by ink, the temperature rise in these portions is large. Depending on the print pattern, it sometimes occurs that some channels do a larger number of times of ink jetting and other channels do a smaller number of times of ink jetting. Because heat generation is larger in channels of a larger number of times of ink jetting, the viscosity of ink in those channels gets lowered and the speed of jetted ink drops becomes faster. As described in the above, if the speed of jetted ink drops varies from one channel to another channel depending on the print pattern, fluctuation of the landing position of the ink drops landing on a medium which is moving at a constant speed with respect to the print head is produced, which lowers image quality remarkably. This is called a cross-talk depending on the print pattern. Because this print head has a shape such that a piezoelectric element is kept between electrodes, it has the same structure as a capacitor, which makes the portions that do not contribute to ink jetting have a large capacitance; therefore, the deformation of the partition walls by a shearing force cannot respond to a high-frequency signal. That is, the structure of a conventional print head is not suitable for a head to have a high image quality, a high speed, and a high density.
A print head of the present invention is characterized by having only straight grooves from which portions not to contribute to jetting are removed. As compared to the conventional one, heat generation is small, and high-frequency response is good. However, because it has neither shallow groove portions nor uncut portions, there no space to form connection electrodes. Connection electrodes must be formed in such a cubic way that they are made to turn around the side surface to the rear surface, where they are connected to flexible cables.
The present pattern formation technology employing a photosensitive resist can form a planar pattern, but cannot form a cubic pattern. Cubic wiring used in a print circuit board or in a dashboard of automobiles is such a coarse one as to have a pitch of mm order. Cubic wiring in the present invention should be such one that, for a 300 DPI head, 70 to 520 stripes of electrodes are formed at a pitch of 80 μm in a cubic way.
The present invention provides a new method of forming a cubic pattern using an electroless plating process and a laser without using a resist.
For a high-speed print head of a high image quality, it is essential to form straight grooves having no useless portions as described in the above. However, if straight grooves are formed, a conventional method of patterning by means of a photosensitive resist cannot be used, because connection electrodes must be formed in a cubic way.
Concerning a technology to form cubic connection electrodes in an ink jet print head, for example, in a technology disclosed in the publication of the unexamined patent application H7-132589 (corresponding to U.S. Pat. No. 5,646,661), straight horizontal grooves are formed and vertical grooves are formed from one end of them respectively. Next, after processing of electroless plating, the vertical surface is ground to remove the plated metal, but plated metal layer remains in vertical grooves to become a part of connection electrodes. Because the piezoelectric element is very hard and fragile, and further, because a high-density head has a wall thickness of 50 μm or under, the partition walls of the grooves are easy to be chipped when the horizontal and vertical grooves are cut and ground.
In the publication of the unexamined patent application 2000-141653, it is disclosed a method in which the rear wall of a print head having straight grooves is masked by a photosensitive resist in a pattern shape, and then electrodes and wiring are formed by evaporation coating. However, because the thickness of the head is as thin as 2 to 10 mm, it is difficult to form a minute pattern by coating a photosensitive resist in this portion to be exposed and developed. In the publication of the unexamined patent application H10-766669, it is disclosed a technology to form a part of connection electrodes by providing a through hole in the bottom of each groove, filling it with a conductive substance. In manufacturing piezoelectric elements, even though exact holes are formed in a green sheet, when it is sintered, expansion and contraction occurs remarkably, which makes it difficult to form holes having a diameter of several tens μm with an accuracy of several μm. Further, when grooves are worked with their position adjusted to coincide with these via holes, it is produced a problem that the positions are deviated for example. In the publication of the unexamined patent application 2000-168094, it is disclosed a method in which the whole of a head having straight grooves is plated, and then, a part of the plating layer is removed by a laser beam, to form connection electrodes. In the publication of the unexamined patent application H8-300667, it is disclosed a method in which electrodes are formed on the inner surfaces of grooves and electrodes formed on the bottom surface of the grooves are removed by a YAG laser. Because a plated metal, for example, nickel is very hard, a large laser power is necessary in order to remove it by evaporation; because laser working is pulse application for a short time of several hundreds nS or under, only the surface layer is rapidly heated to evaporate nickel, and influence to the part under the surface layer is small. However, if the film thickness is large, repeated irradiation is necessary, and there is a possible risk of the deterioration of the underlying piezoelectric element which is weak against heat.
The present invention is a method in which straight channels are formed in a piezoelectric substrate, and there are formed electrodes inside the channels and a cubic wiring for connection outside the channels by an electroless plating process or by an electrolytic plating process and a laser. It is a new method to form electrodes and cubic connection electrodes using plating and a laser without using a photosensitive resist.
The present invention is a new method of forming electrodes and cubic connection electrodes in an ink jet print head by electroless plating or by electrolytic plating and a laser. Up to now, for example, according to the publications of the unexamined patent applications 2000-168094 and H8-300667, after the whole of a print head is plated in an electroless way, unnecessary portions of the plating layer are removed by a laser beam; however, because the plated metal, for example, nickel is a very hard metal, a large laser power is necessary, and there is a possible risk of the underlying piezoelectric element being overheated. When a piezoelectric element is heated to the Curie temperature or over, for example 200 to 300° C., it loses the polarization to be brought into a state of exhibiting no piezoelectricity. Hence, it is necessary to suppress the temperature rise under the half of the Curie temperature. It sometimes happens that when a hard nickel layer on a piezoelectric element is evaporated by a laser beam, the temperature of the piezoelectric element reaches several hundreds degrees locally.
These prior arts are all methods of forming a pattern by evaporating useless parts of plated metal by a laser beam after the metal is plated in an electroless way. Because, in the present invention, a pattern is formed by a laser before electroless plating or on the way of electroless plating, only a low laser power is required, and no excessive heat is applied to the underlying piezoelectric element.
Electroless plating is a method of depositing a metal layer on a support body having no electrical conductivity by chemically reducing a solved metallic salt. Because it is not based on electrolytic reduction which occurs in electrolytic plating, reduction ability is not so strong, that a metal is not deposited solely by it; therefore, it is necessary to make a plating catalyst adsorbed beforehand on a supporting body. By electroless plating, metal can be deposited only on a plating catalyst layer, but once metal is deposited, the plating layer grows because the metal has a self-catalyzing function. The outline of a method of forming electrodes and connection electrodes and a method of assembly of a head based on a conventional technology will be explained by referring to FIG. 15(a) and FIG. 15(b). A plurality of straight grooves 110 are formed in a polarized piezoelectric substrate. Next, a plating catalyst is adsorbed by this head substrate, and electroless plating is carried out. In this state, the whole of the head substrate is uniformly plated. By grinding the front end 111 of the head substrate, that is, the surface to which a nozzle plate is bonded, the plated metal is removed. Next, by applying a laser beam to the part passing from the roof 112 of the partition walls of the grooves, through the rear end of the head 113, to the bottom surface of the head 114, the plated metal on the above-mentioned line-shaped area is removed; thus, electrodes 115 and connection electrodes 116 which are independent for each of the grooves can be formed. Subsequently, by bonding a top plate with an adhesive, the grooves are covered, and by bonding a nozzle plate, an ink inlet plate and a manifold, and connecting flexible wires to the bottom surface of the head, an ink jet print head is completed. In another way, the plated metal on the part of the roofs 112 of the partition walls may be removed by grinding. The present invention is an improvement of this process. Because a part of a plating film is removed before the thick layer is formed or on the way of plating not after the thick film has been formed, it is a method in which only a small power for a laser beam is necessary for a short time, and thermal influence to the piezoelectric element is small.
It is an object of the present invention, by eliminating the various defects of conventional technologies, to provide a method of manufacturing an ink jet print head which is capable of manufacturing at a low cost a high-density print head having a high image quality, a high printing speed, and little cross talk and being small-sized and capable of being driven by a low electric current.